Conventional optical glasses with the optical properties claimed here (heavy flint and extreme barium heavy flint properties) for the application fields of imaging, sensors, microscopy, medical technology, digital projection, photolithography, laser technology, wafer/chip technology, and for telecommunication, optical communication engineering and optics/lighting in the automotive sector, generally contain PbO in order to achieve the desired optical properties, i.e. a refractive index nd of 1.60≦nd≦1.80 and/or an Abbe number νd of 30≦νd≦40. PbO is also frequently used to adjust a pronounced short flint character. This makes these glasses less chemically stable. As2O3 is often used as a refining agent in this case. Since the glass components PbO and As2O3 have been regarded as environmentally unfriendly for some years, most manufacturers of optical instruments and products tend to use lead- and arsenic-free glasses in preference. For use in high price range products, glasses with increased chemical stability are also constantly gaining importance.
Known lead-free glasses with these optical properties are generally based on using TiO2 in a silicate matrix, which leads to glasses which on the one hand are susceptible to crystallization and therefore often not workable in a secondary hot forming step, and on the other hand very difficult to process mechanically owing to high hardness.
In high-quality optical systems, the correction of chromatic aberration is already an important topic from the design of the system onwards. For excellent chromatic aberration correction, glasses with a pronounced short flint character are indispensable. These are glasses whose relative partial dispersion differs from the normal grades and which are therefore particularly suitable for correcting chromatic aberration. This optical property is often due to the use of PbO, however, which is ruled out for the aforementioned ecological reasons.
Instead of the hitherto customary machining of optical components from glass in block or ingot form, production methods in which direct pressings, i.e. blank-pressed or precise pressed optical components and/or preforms for re-pressing which are as close as possible to the final contour, so-called “precision gobs”, can be obtained directly at the end of melting the glass have recently been gaining importance both for the consumer market and for the high-quality sector. “Precision gobs” generally means preferably fully fire-polished, semifree- or free-formed glass portions, which can be obtained via various production methods. For this reason the need for “short” glasses, i.e. for glasses whose viscosity changes very strongly with temperature, has been reported more and more in the context of melting and hot forming process technology. This method has the advantage that it is possible to reduce the hot forming times, and therefore the mould closure times, in precision hot forming close to final geometry. In this way on the one hand the throughput and therefore the time yield can be increased, and on the other hand the mould material is thereby spared, which has a very positive effect on the overall production costs. Furthermore, owing to the faster solidification of short glasses, it is also possible to work glasses with a stronger susceptibility to crystallization, and pre-nucleation, which could be problematic in later secondary hot forming steps, is avoided or at least drastically reduced. For the same reason, there is likewise a need for glasses whose temperature-viscosity profile in absolute terms comprises low temperatures in the hot forming range. Through lower process temperatures, this also contributes to increased mould lifetimes and, through fast stress-free cooling, to low pre-nucleation rates. This also offers a greater range of potentially more cost-effective mould materials, which is significant particularly in precision hot forming close to final geometry.
In modern high-performance optics, ever-greater requirements are also being placed on the imaging accuracy and resolution. This means that on the one hand ever-larger imaging and projection surfaces are being achieved, but on the other hand the structures to be imaged must be ever-smaller and imaged ever-more accurately in respect of point and detail. For this reason it is necessary to illuminate with ever-shorter wavelengths, i.e. with high-energy light, which increases the energy load on the optical elements. In a multiplicity of technical applications, for example microlithography, ever-shorter exposure times are furthermore required in order to increase the production rate, so that the radiation power or beam density which is fed through the optics, i.e. the radiation load per unit time, must necessarily increase. In optical systems, particularly in communications engineering and telecommunications, it is furthermore desirable to obtain a high luminous efficiency, i.e. a high transmission.
This places great demands not only on the development of the respective optics, but also on the glass used for the optics. For example, it is known that the use of high energy densities leads to a phenomenon referred to as solarization, i.e. a radiation-induced modification of the internal structure of the glass, which drastically reduces the transmission i.e. the radiation transparency of an optical element. Glasses which have a high stability against solarization are therefore needed.
The prior art relevant to the invention is summarized in the following documents:                JP 60-221 338 Ohara        DE 3 420 306 Hoya        DE 2 655 857 Hoya        JP 52-045 612 Sumita        
According thereto, it is possible to produce glasses with similar optical properties or comparable chemical composition, although they show significant disadvantages in direct comparison with the glasses according to the invention:
JP 60-221 338 describes lanthanum borate glasses. Here La2O3, which intrinsically absorbs at the blue spectral edge, is used to achieve the desired refractive index properties. Besides the strong susceptibility to crystallization inherent in this glass family, these glasses therefore have a significantly reduced transmission in the blue spectral range relative to La2O3-free glasses.
DE 3 420 306 describes niobium silicates containing high levels of alkaline-earth metal oxides, whose network is destabilized by the high alkaline-earth metal oxide content, here especially CaO at 8 to 42 wt. % and (CaO+MgO) at 16 to 42 wt. %. This leads to strong repercussions on the viscosity-temperature profile, and therefore to glasses which are too short for hot forming close to final geometry and/or secondary hot forming. Furthermore, in order to achieve the high refractive index properties despite large amounts of low-index alkaline-earth metal oxides, larger amounts of high-index but nucleating components have to be tolerated.
DE 2 655 857 likewise discloses niobium silicates, but with different optical properties (lower refractive index and dispersion). High alkaline-earth metal oxide contents (e.g. CaO 5-35 wt. %) can therefore be tolerated without having to use larger amounts of high-index but nucleating components in order to maintain the refractive index.
JP 58-045 612 describes niobium silicates optionally containing up to at most 5 wt. % boron oxide. These low contents do not exhibit the stabilizing effect against crystallization, which is needed for hot forming close to final geometry and/or secondary hot forming, in the niobium silicate glass system destabilized by high levels of high-index components.
It is therefore an object of the present invention to provide an optical glass in a composition range with which the desired optical properties are made possible even without using PbO because of ecological considerations, and as far as possible also without As2O3, Bi2O3 and La2O3, with a reduced TiO2 content. Together with a pronounced short flint character, these glasses should have excellent crystallization stability. These glasses should as far as possible be workable via a precise pressing method and have low transition temperatures Tg. They should furthermore be readily meltable and workable, and have a sufficient crystallization stability which permits manufacture in continuously run plant. A glass which is as short as possible in a viscosity range of 107.6 to 1013 dPas is furthermore desirable. They should be suitable for use in the application fields of imaging, sensors, microscopy, medical technology, digital projection, photolithography, laser technology, wafer/chip technology, and for telecommunication, optical communication engineering and optics/lighting in the automotive sector.